The present invention concerns a process for the production of iron (III) orthophosphate of particularly high purity, an iron (III) orthophosphate produced by the process and the use thereof for the production of LiFePO4 cathode material for Li-ion batteries, as dietary supplements for mineral enrichment and as a molluscicide.
Iron phosphates are used in many areas, for example as dietary supplements or as a nutritional supplement for mineral enrichment, as an active substance in molluscicides, in the ceramic industry or as a raw material for the production of LiFePO4 cathode material for Li-ion batteries. In that respect each area of use makes individual demands on the iron phosphate, while in some uses in particular the chemical purity is of particular importance. In many cases the morphology or particle fineness of the iron phosphate also involves a critical significance for success with the application, for example when considering bioavailability for organisms.
Rechargeable Li-ion batteries are wide-spread power storage devices, in particular in the field of mobile electronics, as the Li-ion battery is distinguished by a high energy density and can supply a high rated voltage of 3.7 volts so that, with a comparable power output, the Li-ion battery is markedly smaller and lighter than conventional batteries. Spinels such as LiCoO2, LiNiO2, LiNi1-xCoxO2 and LiMnnO4 have become established as cathode materials. To increase the reliability and safety of the Li-ion batteries, in particular in relation to thermal overloading in operation, LiFePO4 was developed as the cathode material. That material is distinguished by better power output, higher specific capacitance and high thermal stability in operation.
High purity demands are made on the cathode material of a battery as any contamination which can involve unwanted redox reactions during operation (charging or discharging) detrimentally influences the power of the battery. The nature and concentration of the possible contaminations substantially depends on the quality of the raw materials used for production of the cathode material. Measures for subsequently reducing impurities can be implemented in the cathode material production process, which however is generally linked to an increase in the production costs. It is therefore desirable to use raw materials or starting materials which are as pure as possible for production of the cathode material.
A starting material for the production of LiFePO4 for lithium ion batteries is iron orthophosphate, whose purity and structure or morphology substantially influences the quality of the cathode material produced therefrom.
Known processes for the production of iron (III) orthophosphate use FeSO4 and FeCl3 as starting materials or raw materials, but also metalorganic precursor compounds such as FeC2O2 (Gmelins Handbuch der anorganischen Chemie, Eisen Part B, pages 773 ff; U.S. Pat. No. 3,407,034; C Delacourt et al, Chem Mater 2003, 15 5051-5058; Zhicong Shi et al, Electrochemical and Solid State Letters 2005, 8, A396-A399). The phosphorus or phosphate components in those starting materials are introduced by way of a phosphate salt or phosphoric acid. The described processes also always require additions of HCl, NaOH, NH3, NaClO3 or surfactants in order to control the chemical-physical properties of the products obtained. The consequence of this is that the materials produced in that way always contain impurities of anions such as chloride or sulphate, cations such as sodium or ammonium, or organic compounds. On a large technical scale, those impurities can be removed if at all only by highly complicated and cost-intensive purification processes.
Further cationic contaminations such as for example transition metals which were originally contained in the raw materials used such as FeSO4 or FeCl3 can generally not be easily separated out or washed away as they also generally form phosphate salts which are difficult to dissolve and they crystallise jointly with the desired iron phosphate.
WO 02/30815 describes a process for the production of LiFePO4 from iron phosphate and lithium, wherein an iron oxide is dissolved with heating in 85% phosphoric acid to produce the iron phosphate. The solution is then diluted until the solubility limit of FePO4 is reached and the material crystallises. In that case, unwanted metal phosphates which have a smaller solubility product than FePO4 are to be separated off by fractional dilution. That process suffers from the disadvantage that it requires a very high energy usage and needs a great deal of water to precipitate the product. That process involves the formation of a soluble complex of iron which is stable over weeks and which only slowly crystallises. That considerably reduces the commercial yield of the product. The yield can be increased by boiling the solution over several days, which however requires a very high application of energy. In addition the process involves the occurrence of a large amount of diluted phosphoric acid which can be introduced into the process again only after concentration thereof. The process is therefore not an attractive one both from economic and also ecological points of view.
The processes according to the state of the art for the production of iron phosphates have further disadvantages if the iron phosphate product obtained is to be used for the production of LiFePO4 for Li-ion batteries. Important aspects in terms of suitability of the material are the morphology and the grain size distribution of the iron phosphates. Generally the processes of precipitation of iron phosphate in accordance with the state of the art result in generally spherical crystals of differing sizes. It will be noted however that they have a small surface area in comparison with other crystal morphologies. For use as a cathode material in Li-ion batteries an iron phosphate having a large crystal surface area is advantageous to ensure penetration of the lithium ions in large numbers and at high speed. In addition it is advantageous to produce crystals of small size to reduce the diffusion paths and times of the lithium ions. Furthermore a high bulk density and compressibility of the material is desirable to implement a high energy storage density in the cathode material produced.
Some of the aforementioned disadvantages and problems in the state of the art are overcome by an iron orthophosphate and a process for the production thereof in accordance with parallel-pending German patent application DE 10 2007 049 757. In that process oxidic iron (II)- iron (III)- or mixed iron (II, III) compounds are reacted with phosphoric acid with a concentration in the range of 5% to 50% and iron (II) possibly present after the reaction converted into iron (III) by the addition of an oxidising agent and solid iron (III) orthophosphate is separated from the reaction mixture. Iron (III) present in the starting material is precipitated directly as iron (III) orthophosphate by the addition of the phosphoric acid. The process however suffers from the disadvantage that in part the raw materials and the product are always present side-by-side as solid materials in the course of the reaction. As a result separation of impurities either as a solution or as solid materials is not possible. To achieve a high level of chemical purity for the product it is therefore necessary to rely on and establish the quality and purity of the raw materials.
The object of the present invention was therefore that of providing an iron (III) orthophosphate and a process for the production thereof, in which the known disadvantages from the state of the art are overcome and with which iron (III) orthophosphate can be obtained in a high state of purity in a simpler manner than known production processes.